The present invention relates to sinterable and sintered structures which are microporous and self-supporting.
Sintered, porous materials made of, for example, ceramics or metals, have many uses in a variety of fields, including the fields of industry and medicine. Certain porous sintered structures can serve as filters, for example, whereas others may serve as bone substitute materials. Sinterable particles of various types may be molded, with or without the presence of organic binders, into a desired shape and then fired to remove the binder and to cause the particles to flow into each other at their points of contact. Preformed polymeric structures such as sponges and the like may be infused with a ceramic slip, dried, and fired to remove the organic support and to sinter together the ceramic particles, all for the purpose of manufacturing materials having reasonably large size pores and which are suitable for use as bone substitute materials. Reticulated structures of this kind have the advantage over other porous structures in that there is a three dimensional open continuum without openings being blocked. Thus, full advantage is taken for any merit that porosity confers such as having a fully available internal surface area.
In conventional sintered materials such as ceramic bodies, structures free of pores are normally desired. In the early stages of sintering or consolidation, pores in the structure are largely open from one pore to the next, but the structure is weak. As sintering proceeds, the pores become increasingly smaller and eventually are closed as they are filled by various sintering mechanisms such as diffusion. When a sintered body is about 90% dense, nearly all the pores are closed and thus are isolated from one another.
In general terms, the strength of a sintered body increases as porosity decreases. Conventional methods of making porous bodies involve the use of foaming agents, or the use of particulate materials that can be burned out, but interconnections between the resulting pores are usually far from complete. Connections leading to a more open porosity increase with greater pore formation; hence, resulting in less solid materials with thinner walls and leading to weak structures. These processes have been applied to making closed pore media useful for thermal insulation. Where very fine pores are needed, strength is sacrificed as is the degree of porosity available.
We have discovered, surprisingly, that a strong sintered article can be made that has a structure remarkably similar to conventional reticulated structures (that is, made up of a network of interconnected struts that form a strong, open, interconnected continuum of pores) but in which the struts and openings are orders of magnitude smaller than those of the conventional reticulated structures. The article thus provides a substantially greater open and available surface area than do conventional porous ceramics, and exhibits unexpectedly very fine pores for a given density or strength of the basic material. The article also provides a large network of open channels for contact of these internal surfaces with fluid or gaseous media in processes such as adsorption or catalysis.
Certain polymers such as cellulose can be dissolved in such solvents as N-methylmorpholine-N-oxide to form very viscous, shapeable sols. As an example, a viscous sol of cellulose and N-methylmorpholine-N-oxide (hereafter sometimes referred to as xe2x80x9cNMMOxe2x80x9d), when contacted with water or alcohols with which the NMMO is miscible but in which the cellulose is not soluble, will coagulate as the NMMO is extracted from the sol, leaving behind an open cellulose structure. The viscous sol can be formed into various shapes, such as fibers, before coagulation occurs; coagulation occurs substantially instantly upon contact with water to regenerate the cellulose in the desired shape. Reference is made to U.S. Pat. No. 4,246,221 (McCorsley), U.S. Pat. No. 4,416,698 (McCorsley), U.S. Pat. No. 5,252,284 (Jurkovic et al.) and U.S. Pat. No. 5,540,874 (Yamada et al.). See also U.S. Pat. No. 3,508,941 (Johnson). We further are aware that it has been proposed to add a variety of materials such as flavors, colors, etc. to the thus regenerated cellulose in the shape of beads, or materials such as enzymes to cellulose beads formed by other solvent systems as shown in Tsao et al, U.S. Pat. No. 4,090,022.
The present invention provides a strong, self-supporting, microporous (e.g., having pore sizes in the 0.1-10 micron (xcexc) range) reticulum-like open sintered structure having a variety of uses. The invention also pertains to methods for manufacturing the structure in which the shape of the structure can be readily formed prior to the sintering step.
Briefly, we have found that a sinterable power such as a ceramic or metal can be mixed into a sol of a polymer such as cellulose in a primary solvent such as an amine oxide solvent, (NMMO, for example), following which the mixture can be coagulated to form a gel by contact with a secondary liquid in which the polymer is not soluble and which extracts and replaces the primary solvent, leaving behind a fine, open polymeric network having the sinterable powder arranged in it. The material can be appropriately shaped into fibers, sheets, tubes, rods, etc. either in the form of the viscous sol (by, e.g., extrusion of the viscous, sinterable powder-containing sol into the second, non-solvent for the polymer), or the gel comprising the open, polymeric network having the sinterable powder arranged in it and containing the secondary liquid, such as water, can be suitably formed as by molding to produce the desired shape.
By raising the temperature of the shaped object, the secondary liquid is driven off, enabling the shaped object to shrink if this is desired. Continued heating causes the polymer to be removed, as by pyrolysis, leaving behind it a network of sinterable particles in contact with one another, and, finally, raising the material to sintering temperatures causes the particles to sinter into a porous, self-supporting reticulum.
The resulting sintered product has an outer surface defining a shape having a bulk volume and having interconnecting openings extending throughout the volume and opening through the surface.
Thus, in one embodiment, the invention pertains to a method of producing a porous, sintered structure having an outer surface defining a shape having a bulk volume and having interconnecting openings extending throughout the volume and opening through the surface, comprising of a series of steps, including:
a. preparing a viscous mixture comprising a sinterable powder dispersed in a sol of a polymer in a primary solvent,
b. replacing the primary solvent with a secondary liquid in which the polymer is insoluble to produce a gel comprising an open polymeric network having the sinterable powder arranged therein,
c. removing the secondary liquid from the gel; and
d. sintering the sinterable powder to form the open, porous structure.
In a preferred form of the invention, the method includes the step of forming the material into a predetermined shape, either when the material is in the form of the viscous sol containing the sinterable powder, or, later, when the primary solvent has been replaced by the secondary liquid. When the viscous sol is thus shaped, cooling of the shaped sol before performing step (b) may serve to increase the dimensional stability of the product and reduce distortion when the primary solvent is replaced with the secondary liquid.
In another embodiment, the invention pertains to a shaped, porous product having an outer surface defining a shape having a bulk volume and having interconnecting openings extending throughout that volume and opening through the surface, the article being formed by a method comprising forming into a predetermined shape a shapeable, viscous mixture comprising a sinterable powder dispersed in a sol of a polymer in a primary solvent, and replacing the primary solvent with a secondary liquid in which the polymer is insoluble to produce a gel comprising an open polymeric network having the sinterable powder arranged therein. The secondary liquid is removed from the gel, and the gel is heated to remove the polymeric network and to sinter the sinterable powder to form the shaped open porous structure. Alternatively, the product may be formed by a similar method in which shape forming, as by molding, is performed on the gel after the primary solvent has been replaced with the secondary liquid.
As described above, certain of the porous products of the invention are not only strong and self supporting but in addition present a large available surface. Hence, in another embodiment, the invention relates to a sintered porous article having an outer surface defining a shape having a bulk volume and having a plurality of open, interconnecting pores extending throughout said volume and opening through said surface, the article thus providing a large available surface area on the order of about 1.0 square meters per gram or greater.
In yet another embodiment, the invention relates to a formable material that is capable of being sintered to produce a porous, self-supporting structure. The formable product comprises, in combination, a viscous sol of a polymer dissolved in a primary solvent and which, when contacted with a secondary liquid in which the primary solvent but not the polymer is soluble, forms an open polymeric network. The material includes a sinterable powder dispersed in the viscous sol.
In yet another embodiment, the invention relates to a formable product capable of being sintered to produce an open, porous, self-supporting structure having an outer surface defining a shape having a bulk volume and having interconnecting openings extending throughout said volume and opening through said surface, comprising, in combination, a viscous sol of a polymer in a primary solvent, the weight ratio of polymer to primary solvent not exceeding about 1/10, and a sinterable powder arranged in the sol.
In a further embodiment, the invention relates to a formable product capable of being sintered to produce an open, porous, self-supporting structure having an outer surface defining a shape having a bulk volume and having interconnecting openings extending throughout said volume and opening through said surface, comprising, in combination, a gel formed of a polymer in a secondary liquid in which the polymer is insoluble, the weight ratio of polymer to secondary liquid in the gel not exceeding about 1/10, and a sinterable powder arranged in the gel.
xe2x80x9cSolxe2x80x9d, as used herein in connection with the viscous material resulting from the dissolution of a polymer such as cellulose in a primary solvent such as NMMO, refers to a fluid in which polymeric colloids, e.g., microfibrils of cellulose, are dispersed in the primary solvent. The sol may be cooled or coagulated into a gelxe2x80x94a coherent mass that is solid or jelly-like. When the primary solvent is replaced with the non-solvent secondary liquid to cause coagulation of the sol in the instant invention, a gel results. Particles of the sinterable powder are held in the resulting network of microfibrils. The polymer network is removed, the product is heated, and the particles sinter together to form the unique microporous, microreticulated structure of the invention.
An interesting part of the method of the present invention involves forming an expanded, open, polymer structure within which is dispersed a sinterable powder with the powder being supported by the polymer structure in such a fashion that as the polymer structure is heated, with such concurrent shrinkage as may occur, the sinterable powder particles appear to substantially retain their positions relative to each other, the particles eventually becoming sintered to each other as sintering temperatures are reached. In the case of sinterable ceramic or metal particles which sinter at high temperatures, the polymeric network holding the dispersed particles will tend to shrink as the polymer itself is removed through pyrolysis, but it has been found that the sinterable particles, rather than being released from one another, are drawn together so that when sintering temperatures are reached, the particles themselves will sinter to other particles that they touch to provide the microreticular structure of the invention.
Without being bound by the following explanation, it is believed that the positioning of the particles in the gel, on a microscopic level, is guided at least partially by the positions of the polymeric microfibrils such that the resulting microreticular structure of the sintered product follows to some extent the positions of the fibrils. The resulting product is an open, porous, self-supporting structure having an outer surface defining a shape having a bulk volume and having interconnecting openings or pores extending throughout the volume in a three-dimensional configuration and opening through the outer surface of the product.
The Polymer
The polymer (or blend of polymers, as the case may be) useful in the present invention has several characteristics. First, it must form a sol in a primary solvent, as described above, and must be insoluble in a secondary liquid which is miscible with the primary solvent. Without being bound to the following explanation, it is believed that the polymer, as dissolved, largely retains its molecular chain structure, but that the polymer chains become spaced from one another in the primary solvent. The polymer and primary solvent form a sol that has a substantial viscosity, that is, in the range of about 100 to 5000 poises and above. The sol may be substantially solidxe2x80x94a gelxe2x80x94at lower temperatures such as room temperature; that is, it may be comminuted or otherwise cut and shaped, and the viscous sol may be reconstituted upon heating.
When, as is preferred, the polymer is a cellulose, various fractions having different degrees of polymerization may be utilized, either by themselves or in mixtures or blends of fractions, to obtain varying results. Reference is made to U.S. Pat. No. 5,540,874 (Yamada et al.), the teachings of which are incorporated herein by reference. The sol, and the resulting gel, contains only a very small concentration of polymer in comparison to the amount of liquid, and the weight ratio of polymer to liquid in both the sol and gel is not greater than about 1/10, and desirably is in the range of about 1/100 to about 7/100. The polymer network of the sol and the resulting gel thus is very open.
Preferred polymers are those which can exist in a somewhat fibrous form capable of forming gels, and these include such polysaccharides as cellulose, chitin, and starch. Other potential polymers include the oxygen containing polymers such as poly(vinyl alcohol) and nitrogen bearing polymers such as polyamides, poly(vinyl pyrrolidones) and various polypeptides such as wool, silk, and gelatin. It is believed that the chains of polymers useful in the present invention interengage with one another through hydrogen bonding, as, for example, is the case with cellulose, which hydrogen bonding is interrupted in the presence of a suitable primary solvent. It will be understood, however, that the present invention can utilize any polymers, including polymer blends, which have the characteristics set out above, that is, that it is soluble in a primary solvent to form a viscous sol, insoluble in a secondary liquid that is miscible with the primary solvent to form a gel, and that upon extraction of the primary solvent from a viscous sol of the polymer, the polymer exists as an open, polymeric network.
Of the various polymers that have been considered, cellulose is very suitable, and fibrous cellulose, as is found in cotton, for example, and powdered cellulose (cellulose fibers with a smaller aspect ratio) have given acceptable results.
Solvents
The primary solvent and the secondary liquid, are chosen in connection with the polymer that is employed. As mentioned above, the polymer must be soluble in the primary solvent, must be insoluble in the secondary liquid, and the primary and secondary liquids must be miscible so that the secondary liquid can replace the primary solvent from a viscous sol of the polymer in the primary solvent. By xe2x80x9cmisciblexe2x80x9d, we mean that the secondary liquid and primary solvent are soluble in each other, preferably in most if not all concentrations.
For polysaccharide polymers such as cellulose, primary solvents may include the amine oxide solvents such as N-methylmorpholine-N-oxide, N-methylpiperidine-N-oxide, N-methylpyrrolidine-N-oxide and N-methylazacycloheptane-N-oxide. N,N-dimethylpiperazine may also be appropriate as a primary solvent ingredient. Of the above-listed solvents, N-methylmorpholine-N-oxide (NMMO) containing water in an amount less than about 2 moles of water to 1 mole of NMMO is preferred, and when for simplicity reference is made to NMMO as a primary solvent, it will be understood that this refers to an NMMO/water mixture having a mole ratio of water to NMMO of not greater than 2.
A discussion of NMMO and other solvents for cellulose is found in K. M. Harmon et al., Thermodyanimic and IR study of the hydrates of N-methylmorpholine oxide and quinuclidine oxide. Effect of hydrate stoichiometry on strength of Hxe2x80x94Oxe2x80x94H . . . Oxe2x80x94N hydrogen bonds; implications for the dissolution of cellulose in amine oxide solvents, J. Mol. Struct. (Netherlands) 269, No. 1-2, pp. 109-121 (1992), the teachings of which are incorporated herein by reference.
The purpose of the secondary liquid is to extract the primary solvent from the viscous polymeric sol to coagulate the polymer, now in a liquid in which it is insoluble, as a gel of a highly porous, open polymeric network within which sinterable powder particles are arranged and retained. Although the powder is distributed relatively uniformly throughout the gel on a macroscopic basis, the powder particles on a microscopic basis are positioned and guided by the positions of the microfibrils of the gel. Shrinkage of the product upon removal of some or all of the secondary liquid can be largely controlled by controlling concentrations and form (e.g., microagglomerates) of the various ingredients and by controlling the peak temperature and/or rate at which heating is accomplished.
The polymer network of the gel appears to result in an arrangement of the sinterable particles along and between the microfibers of the network so as to moderate movement of the particles as the gel shrinks; that is, as the gel shrinks, the particles, rather than collapsing into a very close-packed arrangement, appear to maintain to some extent their respective positions, and it is believed that this phenomenon is at least partially responsible for the resulting open, porous structure of the product that results from subsequent sintering.
In connection with celluloses, the secondary liquid ideally is water, which is miscible with NMMO, but which is a non-solvent for cellulose. The secondary liquid may also include alcohols, particularly lower aliphatic alcohols such as methanol, n-propanol, i-propanol and butanol.
It will be understood that the primary solvent and the secondary liquid may actually be mixtures of different liquids. For example, as noted above, a primary solvent for cellulose may include both NMMO and water. Cellulose will dissolve in this primary solvent when the mole ratio of water to NMMO is 2 or less. As noted below, the cellulose may be mixed with a liquid containing the primary solvent and water, e.g., the liquid may contain about 50% to about 70% of NMMO, the remainder being water. Heating of the mixture causes the concentration of the water to decrease as the mixture is heated, the polymer eventually dissolving as the liquid becomes the primary solvent. The secondary liquid may be also a mixture of NMMO and water, but containing a sufficient concentration of water as to render the cellulose insoluble in it. In a continuous procedure for making articles of the invention, NMMO may be recycled, as by stripping it from the secondary liquid, as the concentration of NMMO in the latter rises above a workable level.
In connection with chitin, a suitable primary solvent is dimethylacetamide containing a small amount of lithium chloride. The secondary liquid useful in this regard may be methanol, in which the dimethylacetamide and lithium chloride are miscible, but in which chitin is insoluble.
The Sinterable Powder
As used herein, xe2x80x9csinterablexe2x80x9d refers to the quality of the particulate concentration, the particles of which are in contact, to join to each other at their points of contact to form a coherent mass. Sintering may occur without melting in the case of metal or ceramic particles, or by melting, as in the case with thermoplastic particles. For example, hydroxyapatite powders may be employed having particle sizes in the micron to sub micron range, and mixtures of hydroxyapatite particle agglomerates of several microns in diameter with individual particles in the 0.1 micron size range have given acceptable results.
Among the ceramic powders that may be employed in the current invention are zirconia, alumina, silica, zinc oxide, zeolites, titania, and the various calcium phosphates such as fluorapatite, hydroxyapatite and other tricalcium phosphates. Metal powders include iron, titanium, nickel, cobalt, and others, together with alloys thereof such as CoCrMo alloys. Metal powders sinter much as do the ceramic powders. It is believed that various polymer powders, particularly the thermoplastics, such as polytetrafluoroethylene, having particles that can bond to one another at elevated temperatures, may be appropriate for use in the present invention.
Processing
The first step in the process, of course, is to prepare a viscous mixture of the sinterable powder and the sol comprising the primary solvent and polymer. Although processes of the invention are described below in terms of steps, suggesting batch procedures, it should be understood that the steps can be performed on a continuous basis as well.
Simply put, the polymer and primary solvent are combined in a suitable vessel with stirring. The sinterable powder can be added preferably after the polymer sol has been formed, and stirring is continued until the powder is thoroughly dispersed in the sol. The amounts of primary solvent and polymer are adjusted so that a viscous sol is formed. For a given temperature, greater polymer concentrations and higher polymer molecular weights yield more viscous sols; however, in the present invention, it is desired to minimize the polymer concentration in the sol and resulting gel, and polymer weight concentrations based on the liquid of the sol or the gel generally are not greater than 10% and preferably are in the 1% to 7% by weight range.
Particularly when the polymer tends to form a sol-gel with the primary solvent, as in the case with cellulose and NMMO, viscosity can increase rapidly. In this situation it is believed that the solvent, NMMO, has the ability to break hydrogen bonds readily because of the negatively charged oxygen bound to the nitrogen atom of this solvent. Thus, it may be theorized that when cellulose is dissolved in NMMO, part of the dissolution process involves the breaking of hydrogen bonds between adjacent polymer chains to enable the chains to separate from one another to form a viscous sol of polymer colloids in the NMMO. However, after the sol is formed and is contacted with water to replace the NMMO, hydrogen bonds reestablish themselves between polymer chains, causing the polymer to coagulate and come out of solution inasmuch as the polymer is insoluble in the water.
In a typical procedure, a liquid comprising NMMO at a concentration of about 50% by volume in water may be added to a vessel into which is placed also cellulose powder or small pieces of cellulose. The mixture is stirred, and is heated above the boiling point of water. Water vapor, as steam, is observed to come off, thereby increasing the concentration of NMMO to form a primary solvent at an NMMO concentration of about 75% to about 80% by weight. Once the gradually increasing temperature reaches about 105-106xc2x0 C., a temperature plateau occurs at which water continues to come off, and it is at this point that the cellulose appears to go completely into solution, forming a viscous, light amber clear sol. Desirably, once all the cellulose appears to be in solution and the solution is clear, a suitable amount of sinterable powder, e.g., hydroxyapatite, is added to the sol and is stirred until a uniform, milky-white appearance is obtained, the sol becoming more viscous.
A drop of the viscous sol may now be removed from the container and placed in a beaker of water (the secondary liquid) to form a roughly spherical bead by itself, that is, without application of external shaping forces, as NMMO is extracted from the drop by the water. Gradually, by diffusion, the NMMO throughout the bead is replaced by water. When the process is complete, the bead, which may be on the order of 2 mm in diameter, is a spongy and resilient gel. Microscopic examination of the bead at this point shows an open, porous, fibrous bead surface having the powdered hydroxyapatite distributed through it. The particles may be trapped along and between the cellulose fibers; the fibers may also extend through openings and crevices in clumps or agglomerates of the particles. Many of the cellulose fibrils are intimately entwined around the powder particles so that the particles are securely held in place.
Before or after replacement of the primary solvent with the secondary liquid non-solvent, the mixture may be formed as desired. For example, the highly viscous mixture containing the primary solvent, dissolved polymer and sinterable powder may be forced through an appropriately shaped die into a water bath, the viscous polymer sol coagulating from its surface inwardly as it comes into contact with the water. In this manner, tubes, rods and ribbons of material can be produced with ease. Alternatively, the viscous sol, before being shaped, can be immersed in water to replace the NMMO primary solvent with water, and the resulting somewhat spongy material can be pressed into a suitable form in a mold and pressed into the desired shape.
In any event, the shaped material then can be heated to first drive off water and any remaining NMMO, then to pyrolize the cellulose (which occurs at about 600xc2x0 C.), and thereafter to sinter the finely divided but structured hydroxyapatite particles. Although pyrolysis currently is the method of choice to remove the polymer network, other methods, such as treatment with acid vapors, may also prove valuable.
Although it is explained above for simplicity that the primary solventxe2x80x94primarily NMMO in our examplexe2x80x94is replaced by water, it will be understood that we mean that the NMMO/water primary solvent in which the cellulose is soluble to form a sol is replaced by an NMMO/water solvent containing a much greater concentration of water and in which the cellulose is insoluble, the latter secondary liquid serving to extract NMMO from the primary solvent. The rate of polymer coagulation in this manner can be controlled by regulating the ratio of NMMO to water in the water bath, in the primary solvent, or both.
Of particular interest in connection with many of the products of the invention is the large available surface. Essentially all of the pore surfaces are accessible, since the pores communicate with each other throughout the entire volume of the material. Because of the small pore sizes available, the articles of the invention may be used as filters for filtering from water or other liquids biohazardous microorganisms such as oocysts, proteins or other particulate materials larger than the pores. The pore surfaces, having large surface areas, may serve as supporting surfaces for catalysts. The porous materials exhibit surprising strength for their porosity, and may be used in orthopedic repair applications.